Forecast Center

July/August 2008


This article is a courtesy copy placed on the author's website for educational purposes as permitted by written agreement with Taylor & Francis. It may not be distributed or reproduced without express written permission of Taylor & Francis. More recent installments of this article may be found at the link which follows. Publisher's Notice: This is a preprint of an article submitted for consideration in Weatherwise © 2008 Copyright Taylor & Francis. Weatherwise magazine is available online at:

PART ONE: The Puzzle

While summer and winter often brings stagnant weather patterns, spring is a time of rapid change as intense solar heating clashes with deep polar air masses and cold ocean temperatures. This contrast adds fuel to the atmospheric engine, bringing an assortment of fast-moving systems and changeable weather to the temperate latitudes. In this issue we'll look at a powerful springtime weather system stretched across the Rocky Mountain region.

This weather map is an event during the late afternoon in May. Draw isobars every four millibars (1008, 1004, 1000, 996, etc.) using the plot model example at the lower right as a guide. As the plot model indicates, the actual millibar value for plotted pressure (xxx) is 10xx.x mb when the number shown is below 500, and 9xx.x when it is more than 500. For instance, 027 represents 1002.7 mb and 892 represents 989.2 mb. Therefore, when one station reports 074 and a nearby one shows 086, the 1008 mb isobar will be found halfway between the stations. Then try to find the locations of fronts, highs, and lows.

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Scroll down for the solution

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PART TWO: The Solution

The weather chart for May 12, 2008 shows a potent weather system emerging from the Rocky Mountains. A large, deep mass of cool North Pacific air, represented by the high pressure on the West Coast, covers much of the northern Rockies with temperatures in the 50s and 60s. Meanwhile in the western Gulf of Mexico region, a warm air mass spreads northward into the Corn Belt from the southeastern states.

This particular issue shows an infrequent example of a well-defined front across the Rocky Mountains. Most of the time, fronts lose definition as they enter the mountainous terrain of the western United States. Forecasters must be especially alert and pay close attention to changes in temperature and wind across the region. However in this example there is a striking difference between the warm southwest winds in the Four Corners region and the cool northerly winds in northern Utah and Colorado. Even if this detail is missed, the drawn isobars show a deep trough extending all the way from Colorado to California, hinting at the presence of this front.

In eastern Texas and Louisiana northward, a warm air mass spreads northward. Though it is annotated as "tropical air" for illustrative purposes, it is not true tropical air because of the low dewpoint temperatures. Dewpoint corresponds to the mass of water vapor for a given volume of air, and marine air from the tropics usually has dewpoint readings in the 60s and 70s. The widespread 40s suggests that most of this air mass is in fact polar air that has stagnated over the southeastern United States and the Gulf of Mexico, heated from contact with warm land and water, and is being drawn again northward.

Dewpoint plots also show dry air in the southwest desert region being blown eastward. The portion in west Texas is emerging from the higher elevations of the Rockies and warms adiabatically as it descends, resulting in 90-degree temperatures. On the Great Plains itself, this process is referred to as "downslope warming". When downslope flow involves a specific mountain range rather than a long traverse across gently-descending terrain, the pattern is called a "Chinook", a term common in Colorado and Montana. Regardless of where the wind comes from, high temperature records on the Great Plains are often shattered when strong westerly winds are blowing.

The weather map suggests a boundary between the warm air mass in east Texas and the dry downslope air. This boundary is marked by a dryline, depicted by the brown-colored front. The dryline is often a breeding ground for thunderstorms during the spring and summer since it represents the westernmost extent of tropical air into higher terrain, where it can more easily grow into thunderstorms. The dryline is also the first location where approaching upper-level systems from the west can interact with rich tropical moisture.

Computer programs do not produce the Forecast Center solutions. All fronts and isobars are manually drawn by the author using illustration software.

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